U.S. patent number 5,552,788 [Application Number 08/497,714] was granted by the patent office on 1996-09-03 for antenna arrangement and aircraft collision avoidance system.
This patent grant is currently assigned to Ryan International Corporation. Invention is credited to Dean E. Ryan, Paul A. Ryan.
United States Patent |
5,552,788 |
Ryan , et al. |
September 3, 1996 |
Antenna arrangement and aircraft collision avoidance system
Abstract
An antenna arrangement on a host aircraft for generating power
signals related to a direction from which a transponder reply
signal is received from a threat aircraft. The arrangement includes
first and second monopole antenna elements arranged along a first
axis of the host aircraft, third and fourth monopole antenna
elements arranged along a second axis of the host aircraft, with
the second axis being orthogonal to the first axis, a first
quadrature combiner coupled to the first and second monopole
antenna elements for generating first and second signals from the
received reply signal, and a second quadrature combiner coupled to
the third and fourth monopole antenna elements for generating third
and fourth signals from the received reply signal. The respective
power levels of the first, second, third and fourth signals are
related to the direction from which the reply signal is received
from the threat aircraft.
Inventors: |
Ryan; Paul A. (Dublin, OH),
Ryan; Dean E. (Columbus, OH) |
Assignee: |
Ryan International Corporation
(Columbus, OH)
|
Family
ID: |
23978019 |
Appl.
No.: |
08/497,714 |
Filed: |
June 30, 1995 |
Current U.S.
Class: |
342/30; 342/360;
342/362; 342/455 |
Current CPC
Class: |
G01S
3/30 (20130101); G01S 13/762 (20130101); G01S
3/32 (20130101); G01S 13/767 (20130101) |
Current International
Class: |
G01S
13/76 (20060101); G01S 3/30 (20060101); G01S
13/00 (20060101); G01S 3/14 (20060101); G01S
3/32 (20060101); G01S 013/00 () |
Field of
Search: |
;342/30,455,360,362 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eldred; J. Woodrow
Attorney, Agent or Firm: Pollock, Vande Sande &
Priddy
Claims
What is claimed is:
1. An antenna arrangement on a host aircraft for generating signals
related to a direction from which a transponder reply signal is
received from a threat aircraft, the arrangement comprising:
a first monopole antenna element and a second monopole antenna
element arranged along a first axis of the host aircraft for
receiving the reply signal;
a third monopole antenna element and a fourth monopole antenna
element arranged along a second axis of the host aircraft for
receiving the reply signal, the second axis being orthogonal to the
first axis;
a first quadrature combiner coupled to the first and second
monopole antenna elements for generating first and second signals
from the received reply signal, respective power levels of the
first and second signals being related to the direction from which
the reply signal is received from the threat aircraft; and
a second quadrature combiner coupled to the third and fourth
monopole antenna elements for generating third and fourth signals
from the received reply signal, respective power levels of the
third and fourth power signals being related to the direction from
which the reply signal is received from the threat aircraft.
2. The antenna arrangement according to claim 1, wherein the first
axis is a longitudinal axis of the host aircraft and the second
axis is a lateral axis of the host aircraft.
3. The antenna arrangement according to claim 2, wherein the first
and second monopole antenna elements are located on a first surface
of the host aircraft, and third and fourth monopole antenna
elements are located on a second surface of the host aircraft.
4. The antenna arrangement according to claim 3, wherein the first
surface is a top surface of the host aircraft and the second
surface is a bottom surface of the host aircraft.
5. The antenna arrangement according to claim 3, wherein the first
surface is a bottom surface of the host aircraft and the second
surface is a top surface of the host aircraft.
6. The antenna arrangement according to claim 1, wherein the first
axis is oriented 45.degree. from a longitudinal axis of the host
aircraft.
7. The antenna arrangement according to claim 1, wherein the first
and second quadrature combiners are each a quadrature hybrid
circuit.
8. The antenna arrangement according to claim 7, further comprising
a receiver system coupled to the first and second quadrature
combiners for generating a bearing information signal from the
first, second, third and fourth signals, the bearing information
signal related to the direction from which the transponder reply
signal is received from the threat aircraft.
9. The antenna arrangement according to claim 8, further comprising
a transmitter system coupled to the first and second quadrature
combiners for transmitting a transponder interrogation signal from
the first, second, third and fourth monopole antenna elements.
10. The antenna arrangement according to claim 9, wherein the
transponder interrogation signal is transmitted directionally from
the first, second, third and fourth monopole antenna elements.
11. The antenna arrangement according to claim 7, wherein at least
one of the first and second monopole antenna elements and the first
quadrature combiner, and the third and fourth monopole antenna
elements and the second quadrature combiner are a single unit.
12. An aircraft collision avoidance system on a host aircraft, the
system comprising:
a first monopole antenna element and a second monopole antenna
element arranged along a first axis of the host aircraft for
receiving a transponder reply signal from a threat aircraft;
a third monopole antenna element and a fourth monopole antenna
element arranged along a second axis of the host aircraft for
receiving the reply signal, the second axis being orthogonal to the
first axis;
a first quadrature combiner coupled to the first and second
monopole antenna elements for generating first and second signals
from the received reply signal, respective power levels of the
first and second power signals being related to a direction from
which the reply signal is received from the threat aircraft;
a second quadrature combiner coupled to the third and fourth
monopole antenna elements for generating third and fourth signals
from the received reply signal, respective power levels of the
third and fourth power signals being related to the direction from
which the reply signal is received from the threat aircraft;
and
a receiver coupled to the first and second quadrature combiners for
generating first, second, third and fourth video signals from the
first, second, third, and fourth signals, respectively, the first,
second, third and fourth video signals each having an amplitude
related to the direction from the host aircraft to the threat
aircraft from which the reply signal is received.
13. The aircraft collision avoidance system according to claim 12,
wherein the first axis is oriented 45.degree. from a longitudinal
axis of the host aircraft.
14. The aircraft collision avoidance system according to claim 12,
wherein the first axis is a longitudinal axis of the host aircraft
and the second axis is a lateral axis of the host aircraft.
15. The aircraft collision avoidance system according to claim 14
wherein the first and second monopole antenna elements are located
on a first surface of the host aircraft, and the third and fourth
monopole antenna elements are located on a second surface of the
host aircraft.
16. The aircraft collision avoidance system according to claim 15,
wherein the first surface is a top surface of the host aircraft and
the second surface is a bottom surface of the host aircraft.
17. The aircraft collision avoidance system according to claim 15,
wherein the first surface is a bottom surface of the host aircraft
and the second surface is top surface of the host aircraft.
18. The aircraft collision avoidance system according to claim 15,
wherein the first and second quadrature combiners are each a
quadrature hybrid circuit.
19. The aircraft collision avoidance system according to claim 18,
further comprising:
a first comparator responsive to the first and second video signals
generating a first comparison signal;
a second comparator responsive to the third and fourth video
signals generating a second comparison signal;
a first difference circuit responsive to the first and second video
signals generating a first difference signal;
a second difference circuit responsive to the third and fourth
video signals generating a second difference signal;
a memory loaded with data representing a table of bearing angles as
a function of a pair of comparison signals and magnitudes of a pair
of difference signals; and
means for coupling the first and second comparison signals and the
first and second difference signals to the memory.
20. The aircraft collision avoidance system according to claim 18,
further comprising:
a first analog-to-digital converter responsive to the first video
signal and producing a first digital signal, the first digital
signal representing an amplitude of the first video signal;
a second analog-to-digital converter responsive to the second video
signal and producing a second digital signal, the second digital
signal representing an amplitude of the second video signal;
a third analog-to-digital converter responsive to the third video
and producing a third digital signal, the third digital signal
representing an amplitude of the third video signal;
a fourth analog-to-digital converter responsive to the fourth video
signal and producing a fourth digital signal, the fourth digital
signal representing an amplitude of the fourth video signal;
and
a processor for generating a bearing signal from the first, second,
third and fourth digital signals, the bearing signal being related
to the direction from which the reply signal is received from the
threat aircraft.
21. The system according to claim 20, wherein the processor
comprises:
a first comparator producing a one bit first polarity signal
responsive to the first and second digital signals;
a second comparator producing a one bit second polarity signal
responsive to the third and fourth digital signals;
a first digital difference circuit responsive to the first and
second digital signals generating a first difference signal;
a second digital difference circuit responsive to the third and
fourth digital signals generating a second difference signal;
a memory loaded with data representing a table of bearing angles as
a function of a pair of polarity bits and a pair of difference
signals; and
means for coupling the first and second difference signals and the
first and second polarity signals to the memory.
22. The aircraft collision avoidance system according to claim 12,
further comprising a transmitter coupled to the first and second
quadrature combiners for transmitting a transponder interrogation
signal from the first, second, third and fourth monopole antenna
elements.
23. The aircraft collision avoidance system according to claim 22,
wherein the transponder interrogation signal is transmitted
directionally from the first, second, third and fourth monopole
antenna elements.
24. The aircraft collision avoidance system according to claim 12,
wherein at least one of the first and second monopole antenna
elements and the first quadrature combiner, and the third and
fourth monopole antenna elements and the second quadrature combiner
are a single unit.
25. A method for determining a bearing of a transponder reply
signal received from a threat aircraft with respect to a heading of
a host aircraft, the host aircraft including a first monopole
antenna element and a second monopole antenna element arranged
along a first axis of the host aircraft, a third monopole antenna
element and a fourth monopole antenna element arranged along a
second axis of the host aircraft, the second axis being orthogonal
to the first axis, the method comprising the steps of:
receiving the reply signal at the first, second, third and fourth
monopole antenna elements;
generating first, second, third and fourth received signals, the
first, second, third and fourth received signals related to the
reply signal received at the first, second, third and fourth
monopole antenna elements, respectively;
generating first and second signals from a quadrature summation of
the first and second received signals, the first signal
corresponding to an antenna pattern in a first direction along the
first axis of the host aircraft, and the second signal
corresponding to an antenna pattern in a second direction along the
first axis of the host aircraft;
generating third and fourth signals from a quadrature summation of
the third and fourth received signals, the third signal
corresponding to an antenna pattern of a first direction along the
second axis of the host aircraft, and the fourth power level signal
corresponding to an antenna pattern of a second direction along the
second axis of the host aircraft;
generating first, second, third and fourth video signals from the
first, second, third and fourth signals, respectively, the first,
second, third and fourth video signals each having an amplitude
related to a bearing of the reply signal received from the threat
aircraft with respect to the heading of a host aircraft.
26. The method according to claim 25, wherein the first axis is
oriented 45.degree. from a longitudinal axis of the host
aircraft.
27. The method according to claim 25, wherein the first axis is a
longitudinal axis of the host aircraft, the first direction along
the first axis is in a forward direction of the host aircraft, and
the second direction along the first axis is in an aft direction of
the host aircraft, and the second axis is lateral axis of the host
aircraft, the first direction along the second axis is in a right
direction of the host aircraft and the second direction along the
second axis is in a left direction of the host aircraft.
28. The method according to claim 25 further comprising the steps
of:
comparing the first and second video signals for generating a first
polarity signal;
comparing the third and fourth video signals for generating a
second polarity signal;
generating first difference signal based on a difference between
the first and second video signals;
generating second difference signal based on a difference between
the third and fourth video signal; and
generating a bearing signal based on the first and second polarity
signals and the first and second difference signals, the bearing
signal related to a bearing of the reply signal received from the
threat aircraft with respect to a heading of the host aircraft.
29. The method according to claim 25, further comprising the steps
of:
converting the first, second, third and fourth video signals to
first, second, third and fourth digital signals, respectively, the
first, second, third and fourth digital signals each representing
an amplitude of the first, second, third and fourth video signals,
respectively;
comparing the first and second digital signals for generating a
first polarity bit;
comparing the third and fourth digital signals for generating a
second polarity bit;
generating a first difference signal based on a difference between
the first and second digital signals;
generating a second difference signal based on a difference between
the third and fourth digital signals; and
generating a bearing signal based on the first and second polarity
bits and the first and second difference signals, the bearing
signal related to a bearing of the reply signal received from the
threat aircraft with respect to the heading of the host
aircraft.
30. The method according to claim 29, wherein the step of
generating the bearing signal comprises the step of addressing
bearing data stored in a look-up table in a memory.
31. The method according to claim 25, wherein the first and second
monopole antenna elements are located on a top surface of the host
aircraft.
32. The method according to claim 31, wherein the third and fourth
monopole antenna elements are located on a bottom surface of the
host aircraft.
33. The method according to claim 25, further comprising the steps
of:
generating a transponder interrogation signal; and
coupling the transponder interrogation signal to the first, second,
third and fourth monopole antenna elements for transmitting the
transponder interrogation signal.
34. The method according to claim 33, wherein the step of coupling
the transponder interrogation signal to the first, second, third
and fourth monopole antenna elements generates a directionally
transmitted signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an antenna arrangement. More
particularly, the present invention relates to an antenna
arrangement associated with an aircraft collision avoidance system
on a host aircraft for determining a direction from which a
transponder reply signal is received from threat aircraft.
2. Description of the Related Art
Almost all active aircraft are equipped with a transponder that,
when interrogated, transmit reply signals having coded information
relating to the aircraft associated with the transponder such as
the altitude of the aircraft, for example, Transponder-based
aircraft collision avoidance systems rely on transmitted reply
signals from the existing population of airborne transponders for
operation. By receiving coded transponder reply signals at a host
aircraft that are transmitted from a threat aircraft, the altitude
separation between the host aircraft and the threat aircraft can be
determined and a safe altitude separation can be maintained between
the two aircraft for avoiding a disastrous encounter. A
transponder-based aircraft collision avoidance system is disclosed
in U.S. Pat. No. 5,077,673 and U.S. Pat. No. 5,157,615, both issued
to Brodegard et al., both of which are incorporated by reference
herein.
The effectiveness of a collision avoidance system can be further
enhanced by determining the approximate distance from a host
aircraft to a threat aircraft based on the strength of a received
transponder reply signal. Accurate ranging to a threat aircraft is
accomplished by a host aircraft actively interrogating a threat
aircraft, and then measuring the time between transmission of an
interrogation signal and reception of a reply signal.
Although knowledge of the direction, or bearing, to a threat
aircraft relative to the heading of a host aircraft is not
essential to have collision avoidance, knowledge of the bearing to
a threat aircraft greatly enhances a pilot's ability for visually
acquiring the threat aircraft and for having a better spacial
perspective of threat aircraft relative to the host aircraft.
Directional antennas are used in some transponder-based collision
avoidance systems for determining bearing to a threat aircraft, but
they are specialized and require complex circuitry for reliable
operation. One common arrangement uses a quadrapole antenna array
with output signals being combined so that the phase difference
between two output ports of the combining circuitry are indicative
of the bearing of a received transponder signal. This particular
arrangement requires that the antenna cables be phase matched, and
that the phase matching be maintained through two receiving
channels of the system for accurate and reliable bearing
determination. This requirement is both delicate and complex.
Another problem encountered with conventional aircraft collision
avoidance systems using directional antenna is the location of the
antenna on the airframe. Signals arriving at the antenna from
directions that are "shadowed" by the airframe, that is, from
directions which place the transmitter out of the line of sight
from the antenna by virtue of the airframe blocking the antenna
from the transmitter, are attenuated. Shadowing is presently
overcome by placing a primary directional antenna on a top surface
of the aircraft and a second antenna on a bottom surface of the
aircraft. The bottom mounted antenna is usually omnidirectional.
While using a single directional antenna reduces cost, it only does
so at the expense of reduced directional coverage. To compensate
for this drawback, the bottom antenna can be a duplicate of the top
mounted directional antenna, but use of two directional antennas
adds considerable cost and complexity to the aircraft collision
avoidance system.
SUMMARY OF THE INVENTION
The antenna arrangement of the present invention uses standard
off-the-shelf monopole antenna elements available from a plurality
of sources. By using readily available antenna elements, cost and
availability are substantially improved. Preferably, the antenna
elements are commonly available 1/4-wavelength transponder antennas
arranged in an antenna array that is split between two locations on
an aircraft, preferably the top and the bottom of the aircraft. By
placing part of the antenna array on top of the aircraft and part
on the bottom of the aircraft, airframe shadowing does not result
in complete loss of bearing of a threat aircraft. Further,
installation is simplified.
Additionally, the reception circuitry of the present invention is
significantly simplified, and provides a virtually instantaneous
measurement of bearing and better overall performance than
conventional aircraft collision avoidance systems in determining
bearing from all directions. This is because, after antenna signals
are combined, phase shift is not critical from that point forward
in the system through the aircraft since only signal magnitudes are
utilized for determining bearing. Further, the present invention
provides an aircraft collision avoidance system which can transmit
transponder interrogation signals directionally thus eliminating
dependence on ground-based radar systems for interrogating threat
aircraft transponders.
The present invention provides an antenna arrangement on a host
aircraft for generating signals related to a direction from which a
transponder reply signal is received from a threat aircraft. The
arrangement of one embodiment includes first and second monopole
antenna elements arranged along a first axis of the host aircraft
for receiving the reply signal, third and fourth monopole antenna
elements arranged along a second axis of the host aircraft for
receiving the reply signal with the second axis being orthogonal to
the first axis, a first quadrature combiner coupled to the first
and second monopole antenna elements for generating first and
second signals from the received reply signal with respective power
levels of the first and second signals being related to the
direction from which the reply signal is received from the threat
aircraft, and a second quadrature combiner coupled to the third and
fourth monopole antenna elements for generating third and fourth
signals from the received reply signal with respective power levels
of the third and fourth signals being related to the direction from
which the reply signal is received from the threat aircraft.
Preferably, the first axis is a longitudinal axis of the host
aircraft, the second axis is a lateral axis of the host aircraft,
the first and second monopole antenna elements are located on a top
surface of the host aircraft, and the third and fourth monopole
antenna elements are located on a bottom surface of the host
aircraft. The first axis can also be oriented 45.degree. from a
longitudinal axis of the host aircraft. The first and second
quadrature combiners are each preferably a quadrature hybrid
circuit. At least one of the pairs of monopole antenna elements and
the corresponding quadrature combiner can be fabricated as a single
unit for minimizing phase matching considerations.
One embodiment of the antenna arrangement of the present invention
includes a receiver system coupled to the first and second
quadrature combiners for respectively generating first, second,
third and fourth video signals from the first, second, third and
fourth signals, respectively, with the first, second, third and
fourth video signals each having an amplitude related to the
direction from the host aircraft to the threat aircraft from which
the reply signal is received. Analog-to-digital converters receive
each of the video signals for generating corresponding digital
signals representing amplitudes of the video signals. A first
comparator compares two of the digital signals for generating a
first polarity bit signal. A second comparator compares the other
two digital signals for generating a second polarity bit signal. A
first difference circuit generates a first difference signal from
the first and second video signals. A second difference circuit
generates a second difference signal from the third and fourth
video signals. A processor with a memory containing a look-up table
receives the first and second difference signals and the polarity
bit signals for generating a bearing signal related to the
direction from which the reply signal is received from the threat
aircraft. The present invention can also include a transmitter
system coupled to the first and second quadrature combiners for
transmitting a directional transponder interrogation signal.
The present invention also provides a method for determining a
bearing of a transponder reply signal received from a threat
aircraft with respect to a heading of a host aircraft. According to
an embodiment of the invention, the host aircraft includes first
and second monopole antenna elements arranged along a first axis of
the host aircraft, third and fourth monopole antenna elements
arranged along a second axis of the host aircraft with the second
axis being orthogonal to the first axis. The method includes the
steps of receiving the reply signal at the first, second, third and
fourth monopole antenna elements, generating first, second, third
and fourth received signals with the first, second, third and
fourth received signals related to the reply signal received at the
first, second, third and fourth monopole antenna elements,
respectively, generating first and second signals from a quadrature
summation of the first and second received signals such that the
first signal corresponds to an antenna pattern in a first direction
along the first axis of the host aircraft, and such that the second
signal corresponds to an antenna pattern in a second direction
along the first axis of the host aircraft, generating third and
fourth signals from a quadrature summation of the third and fourth
received signals such that the third signal corresponds to an
antenna pattern of a first direction along the second axis of the
host aircraft, and such that the fourth signal corresponds to an
antenna pattern of a second direction along the second axis of the
host aircraft, generating first, second, third and fourth video
signals from the first, second, third and fourth signals,
respectively, such that the first, second, third and fourth video
signals each have an amplitude related to a bearing of the reply
signal received from the threat aircraft with respect to the
heading of a host aircraft. Preferably, the first axis is a
longitudinal axis of the host aircraft, the first direction along
the first axis is in a forward direction of the host aircraft and
the second direction along the first axis is in an aft direction of
the host aircraft, and the second axis is a lateral axis of the
host aircraft, the first direction along the second axis is in a
right direction of the host aircraft and the second direction along
the second axis is in a left direction of the host aircraft. Also,
the first, second, third and fourth video signals are converted to
first, second, third and fourth digital signals, respectively, such
that the first, second, third and fourth digital signals each
represent an amplitude of the first, second, third and fourth video
signals, respectively. The first and second digital signals are
compared to each other for generating a first polarity bit signal.
Similarly, the third and fourth digital signals are compared to
each other for generating a second polarity bit signal. A first
difference signal is generated from the first and second digital
signals and a second digital signal is generated from the third and
fourth digital signals. The bearing signal is generated based on
the first and second difference signals and the first and second
polarity bits such that the bearing signal is related to a bearing
of the reply signal received from the threat aircraft with respect
to the heading of a host aircraft. The transponder interrogation
signal can be coupled to the first and second quadrature combiners
for transmitting a directional transponder interrogation signal
from the first, second, third and fourth monopole antenna
elements.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is illustrated by way of example and not
limitation in the accompanying figures in which like reference
numerals indicate similar elements and in which:
FIGS. 1(a)-1(d) show two arrangements of pairs of 1/4 wavelength
monopole antenna elements and their corresponding antenna field
patterns with quadrature summation;
FIG. 2 shows a schematic block diagram of a quadrature hybrid
circuit;
FIG. 3 shows a circuit arrangement of two monopole antenna elements
and a quadrature hybrid circuit for forming two unidirectional
antenna field patterns;
FIG. 4 shows a pair of 1/4 wavelength monopole antenna elements
spaced 1/4 wavelength apart and mounted on an infinite ground
plane;
FIG. 5 shows a field intensity pattern for a pair of monopole
antenna elements, after being combined by a quadrature hybrid, and
for a single monopole antenna element;
FIGS. 6(a)-6(d) show two pairs of monopole antenna elements mounted
on an aircraft and the corresponding antenna field patterns for
each pair of antenna elements according to the present
invention;
FIGS. 6(e) and 6(f) show two pairs of monopole antenna elements
mounted on an aircraft with an alternate orientation of orthogonal
axis according to the present invention;
FIG. 7 shows a schematic block diagram of a single receiver channel
for an aircraft collision avoidance system according to the present
invention;
FIGS. 8(a) and 8(b) show the configuration of four receiver
channels for producing antenna field patterns for measuring bearing
according to the present invention;
FIG. 9(a)-9(d) show normalized antenna field patterns for the
antenna arrays of FIGS. 6(a) and 6(b) according to the present
invention for maximum expected signal levels;
FIGS. 10(a)-10(d) show compressed difference patterns for the
antenna patterns of FIGS. 9(a) and 9(d) according to the present
invention;
FIGS. 11(a)-11(d) show a first quadrant representation of
.vertline.F-A.vertline. patterns superimposed on
.vertline.R-L.vertline. patterns according to the present invention
for maximum expected signal levels;
FIG. 12 shows a processing arrangement according to the present
invention for generating a bearing information signal from field
patterns signals; and
FIG. 13 shows a transmitter system coupled to the antenna
arrangement of the present invention for transmitting a directional
transponder interrogation signal.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is directed to an antenna arrangement and an
aircraft collision avoidance system that use two pairs of monopole
antenna elements for determining a bearing of a threat aircraft
with respect to a heading of a host aircraft.
FIG. 1(a) shows a top view of two 1/4-wavelength monopole antenna
elements 11 and 12 mounted on an infinite ground plane and
separated by a 1/4 wavelength spacing. If the signal received by
monopole element 12 is phase shifted 90.degree. by phase shifter 13
and summed with the received signal of monopole element 11 by
combiner 14, the unidirectional field pattern shown in FIG. 1(b) is
produced at the combiner output 15 (no mutual coupling). The
azimuth .phi. of the received signal with respect to the axis on
which antenna elements 11 and 12 are aligned is represented in both
FIGS. 1(a) and 1(b). FIGS. 1(c) and 1(d) respectively show the
situation where the signal received by monopole element 11 is phase
shifted 90.degree. before summation with the received signal of
element 12 and the resulting unidirectional field pattern produced
at combiner output 15. The bearing or azimuth of the received
signal with respect to the axis on which antenna elements 11 and 12
are aligned is represented by .phi..
FIG. 2 shows a schematic block diagram of a quadrature hybrid
circuit 20 utilized by the present invention in connection with an
arrangement of monopole antenna elements for producing the field
patterns of FIGS. 1(b) and 1(d). A quadrature hybrid circuit, also
known as a branch-line hybrid or a hybrid ring, is a 3 dB
directional coupler having a 90.degree. phase difference between
the outputs of the through and the coupled arms of the hybrid. It
is well-known that a quadrature hybrid circuit can be used either
as a power divider or as a power combiner. As shown in FIG. 2, a
quadrature hybrid circuit has four quarter-wave transmission lines
21-24 coupled together in a well-known manner. Each port 25-28 of
hybrid circuit 20 is terminated with a characteristic impedance
Z.sub.0 (not shown) and the output ports 27 and 28 have the
property of being isolated from each other.
If a single monopole antenna element is connected to input port 25,
the output signal power appearing at both ports 27 and 28 resulting
from the input to port 25 will be equal and will have a 90.degree.
phase difference between port 27 and port 28. Since input port 25
and isolated port 26 are interchangeable, a monopole antenna
element connected to isolated port 26 will similarly cause the
output signal power appearing at both output ports 27 and 28 to be
equal and have a 90.degree. phase difference. When a monopole
antenna element is connected to both input ports 25 and 26, two
field patterns corresponding to those shown in FIGS. 1(b) and 1(d)
are simultaneously generated by superposition. FIG. 3 shows a
circuit arrangement 30 of two monopole antenna elements 11 and 12
and a quadrature hybrid circuit 20 for forming two unidirectional
antenna field patterns 31 and 32.
FIG. 4 shows a pair of 1/4 wavelength monopole antenna elements 1
and 2 spaced 1/4 wavelength apart and mounted on an infinite ground
plane. The following equations describing the antenna patterns of
the present invention are normalized to an antenna pattern for a
single 1/4-wavelength monopole antenna mounted on an infinite
ground plane. Only the equatorial-plane (horizontal-plane) patterns
are considered and the various potentials described are with
respect to a phase center 3, that is, halfway between antenna
elements 1 and 2.
Since 1 wavelength equal 2.pi. radians, the electrical potentials
of the monopole elements in phasor notation are:
where, E.sub.1 and E.sub.2 are the electrical potentials of
monopole elements 1 and 2, respectively, with respect to phase
center 3. These two potentials are combined using a quadrature
hybrid (FIG. 2) with the phase of E.sub.2 shifted an additional
90.degree. (see FIG. 1(a)) to give,
The coefficient 1/(2).sup.1/2 is a result of the antenna power
being divided equally between the two outputs ports of the adrature
hybrid. An equation similar to Equation (3) can be written for the
situation where the phase of E.sub.1 is shifted an additional
90.degree. (see FIG. 1(c)).
Expanding Equation (3) into real and imaginary parts,
Simplifying Equation (4) yields,
Equations (3) and (4) are for the electric field pattern of a pair
of monopole antenna elements referenced to a single monopole
antenna element. A polar plot 51 of Equation (5) is shown in FIG. 5
superimposed on a polar plot 52 of a single monopole antenna
element. As shown in FIG. 5, the maximum electric field intensity
for two monopole antenna elements is 2.sup.1/2 times greater than
the intensity of a single monopole antenna element. Thus, the
resulting power gain for the two monopole antenna elements is two
(referenced to a single monopole element), and the half-power beam
width is 180.degree..
FIGS. 6(a)-6(d) show two pairs of monopole antenna elements mounted
on an aircraft and the corresponding antenna field patterns for
each pair of antenna elements. FIG. 6(a) shows a first pair of
monopole elements F and A that are preferably mounted on a top
surface 63 of an aircraft 61 and aligned along a reference axis 62.
In FIG. 6(a), reference axis 62 is aligned along a longitudinal
axis of aircraft 61. Element F is the forward-most antenna element
of the first monopole pair, while element A is the aft-most antenna
element of the pair. FIG. 6(c) shows a second pair of monopole
elements R and L that are preferably mounted on a bottom surface 64
of aircraft 61 and aligned along a second axis 65 that is
orthogonal to reference axis 62. In FIG. 6(c), the second axis 65
is parallel to an axis extending from wing tip to wing tip of
aircraft 61. Element R is the right-most antenna element of the
second monopole pair, while element L is the left-most antenna
element of the pair (referenced to the aircraft in an upright
attitude). The first and second pairs of monopole elements can be
interchanged, that is, elements F and A can be mounted on the
bottom surface 64 of aircraft 61, while elements R and L can be
mounted on the top surface 63 of the aircraft. Further, the axis
along which the two pairs of antenna elements are aligned can be
oriented at another angle relative to the heading of the host
aircraft, for example, oriented 45.degree. from the longitudinal
axis shown in FIGS. 6(a) and 6(c). In such an arrangement, as shown
in FIGS. 6(e) and 6(f) for example, the processing arrangement
coupled to the pairs of antenna elements will need to account for
the alternative orientation of the orthogonally arranged axes on
which the antenna element pairs are aligned.
FIG. 6(b) shows the antenna field patterns for elements F and A
when they are coupled to a quadrature hybrid circuit in the manner
shown in FIG. 3. FIG. 6(d) shows similar antenna field patterns for
monopole elements R and L when coupled to a quadrature hybrid
circuit.
Referring to FIG. 3, the transmission lines 33 and 34 connecting
monopole antenna elements 11 and 12 to quadrature hybrid circuit 20
must be phase matched according to the present invention, but cable
lengths beyond the outputs of hybrid 20 are not critical with
respect to phase considerations because only signal magnitudes are
used by the present invention for determining bearing of a received
reply signal, and not phase relationships. By mounting hybrid
circuit 20 close to monopole elements 11 and 12, the cable
connections between elements 11 and 12 and hybrid circuit 20 will
be short and will, accordingly, not be difficult to phase match
and/or calibrate for removing any phase mismatch. To further reduce
the complexity of the present invention, quadrature hybrid circuit
20 can be fabricated as a single unit with a pair of monopole
antenna elements, thus minimizing any phase matching required by
the present invention prior to combiner 20. In FIG. 3, this would
be achieved by fabricating elements 11 and 12 to connect directly
to ports 25 and 26, respectively, of hybrid circuit 20 omitting
lines 33 and 34.
FIG. 7 shows a schematic block diagram of a single receiver channel
70 for an aircraft collision avoidance system according to the
invention. Preferably, the present invention includes four receiver
channels configured like single receiver channel 70, that is, one
for each field pattern, but fewer receiver channels can be used by
storing information and switching. However, by using fewer than
four receiver channels, the system loses the ability to perform
instantaneous bearing measurements. Preferably, four separate and
identical receivers are used, one for each channel, so that bearing
of a received transponder reply signal can be measured during each
separately received pulse. Typically transponder replies include
pulses having a 450 nanosecond duration. Since the receivers
themselves do not require phase tracking according to the present
invention, and since normal gain variations between receivers cause
no more than an acceptable level of error, four identical receiver
channels are not difficult or costly to fabricate or maintain.
Receiver channel 70 is a conventional receiver channel. Input port
71 is coupled to one of the output ports 27 and 28 of a quadrature
hybrid circuit 20. The output signal of the hybrid circuit is
filtered by bandpass filter 72 that has an RF center frequency of
1090 MHz, for example. The output of filter 72 is coupled to a
first input of mixer 73. A local oscillator signal from local
oscillator (LO) 74 is coupled to a second input of mixer 73 for
down-converting the filtered antenna signal to an intermediate
frequency (IF) signal. The down-convertered output of mixer 73 is
coupled to an IF filter 75 that has a center frequency of 60 MHz,
for example. The output of IF filter 75 is coupled to a logarithmic
detector 76 for producing a video signal. The logarithmic detector
permits operation of the system over a wider dynamic range than a
linear detector. The video output of detector 76 is amplified by
video amplifier 77 and output at channel output 78.
Local oscillator 74 can be common to all four channels of the
collision avoidance system of the present invention for reducing
the total number of functional elements of the system. While
exemplary RF, LO, and IF frequencies are indicated, other
frequencies can be readily used.
FIGS. 8(a) and 8(b) show an arrangement of four receiver channels
81-84 for producing video signals having amplitudes representing
antenna field patterns for measuring bearing according to the
present invention. The antenna field patterns shown have been
conditioned by the logarithmic detectors of each receiver channel.
Each of the receiver channels 81-84 can use the circuit of channel
70 although the LO 74 may be common to all four channels.
To determine the bearing of a received transponder reply signal the
compressed logarithmic signal output from a receiver channel is
normalized to unity, that is, the maximum signal level (minimum
range) is set to 1. The compressed logarithmic output of a channel
has the form:
where, k is a constant.
The only meaningful range of values for the compressed output is
zero and above, so Equation (6) is only applicable for input values
that satisfy this restriction. For all other input values, the
compressed output is zero. The patterns of FIGS. 9(a)-9(d)
illustrate the effects of logarithmic compression on the antenna
patterns of the monopole arrays of the present invention. FIGS.
9(a) and 9(b) show normalized antenna patterns for the top and
bottom monopole antenna arrays of FIGS. 6(a) and 6(c),
respectively, for a maximum expected signal level. FIGS. 9(c) and
9(d) show normalized antenna field patterns for the top and bottom
monopole antenna arrays of 6(a) and 6(c), respectively, for a
minimum expected signal level. The azimuth .phi. of a received
signal with respect to the axis on which a monopole pair is align
is also shown in FIGS. 9(a)-9(d).
Since the patterns are non-linear due to logarithmic compression,
pattern shape is a function of received signal amplitude. The polar
patterns of FIGS. 9(a)-9(d) are based on a 25:1 dynamic range
between the expected maximum and minimum range for the system of
the present invention. It has been found that this is a very
practical range of operation for collision avoidance
applications.
The logarithmic compressed patterns of FIGS. 9(a)-9(d) can be
further combined for useful adaptation. By producing the difference
between the forward F and aft A compression patterns, and the
difference between the left L and right R compression patterns, and
by further preserving magnitudes only, that is, absolute values,
the four patterns are reduced to two patterns. These difference
patterns have a "Bow Tie" shape, and are shown by FIGS. 10(a) and
10(b). Since the difference patterns are also non-linear because of
the logarithmic compression, the pattern shape is a function of
signal amplitude. Again, the patterns shown in FIGS. 10(a)-10(d)
are for the end limits of the 25:1 dynamic range of operation.
The logarithmic compressed signals from the four receiver channels
are digitized using analog-to-digital (A/D) converters for further
processing. Quadrant determination can be made immediately from
these four digitized signals. With the channels identified as F -
Forward, A - Aft, R - Right, and L - Left, if F>A, the bearing
of a received signal is within the forward two quadrants. If
F<A, the bearing is within the aft two quadrants. If R>L,
bearing is within the right two quadrants, and if R<L, bearing
is within the left two quadrants. By generating two sign or
polarity bits, one for the difference between Forward/Aft and the
other for the difference between Right/Left, the quadrant of the
bearing is distinguished. For example, if the two polarity bits are
(+/+) (with the first bit designated for forward/aft and the second
bit designated for right/left), then bearing of a received reply
signal is within the first quadrant. If the two polarity bits are
(-/+), then bearing is within the second quadrant. If the two
polarity bits are (-/-), bearing is within the third quadrant, and
if the polarity bits are (+/-), bearing is within the fourth
quadrant.
The four digitized signals are also combined for forming a set of
two difference signals. The first difference signal is the
.vertline.F-A.vertline. signal representing the magnitude of the
difference between the F-channel and the A-channel. The
.vertline.R-L.vertline. signal represents the magnitude of the
difference between the R-channel and the L-channel. This pair of
difference channel signals, together with the two sign bits for
quadrant identification, provide all necessary information for
determining an accurate bearing. Since the patterns have mirror
symmetry among the respective quadrants, determination of the
bearing angle from a cardinal axis can be combined with quadrant
awareness (polarity bits) for providing an unambiguous bearing
angle.
FIGS. 11(a)-11(d) show a first quadrant representations of the
.vertline.F-A.vertline. patterns superimposed with the
.vertline.R-L.vertline. patterns on the same polar and rectilinear
graphs for maximum and minimum expected signal levels. FIGS. 11(a)
and 11(b) show superimposed patterns for maximum expected signal
levels and FIGS. 11(c) and 11(d) show superimposed patterns for
minimum expected signal levels. The non-linear effects of
compression are apparent from the graphic representations of FIGS.
11(a)-11(d).
Direct mathematical solutions for the transcendental equations
relating the digitized values of a received reply signal to a
bearing angle from which the reply signal is received are not
straightforward. Iterative algorithms are well-known within the
computer arts for solving equations that have no simple converse
solutions. Iterative algorithm computer programs can readily
calculate the bearing angle and total signal magnitude for
determining the bearing angle solution for every pair of digitized
values of .vertline.F-A.vertline. and .vertline.R-L.vertline..
Iterative solutions, however, are inherently slow. Depending upon
the desired accuracy of a solution, the number of iterations for
reaching a single solution can be prohibitively long. To overcome
this drawback, the present invention preferably uses a look-up
table stored in a memory for determining the bearing of a received
transponder reply signal. The look-up table contains the bearing
angle solutions to all possible combinations of digitized values to
a practical resolution. Thus, bearing angle solutions are computed
nearly instantaneously by merely accessing the look-up table stored
in the memory in a well-known manner using the pair of magnitudes
.vertline.F-A.vertline. and .vertline.R-L.vertline. along with the
polarity bits.
The instantaneous nature of bearing acquisition according to the
present invention is of great benefit for collision avoidance, as
well as other related applications. When receiving transponder
reply signals, each reply consists of short multiple pulses in
sequence. The present invention is capable of determining bearing
for each such short pulse. Consequently, there is an abundance of
repetitive bearing information generated by the present invention
which allows for rapid updates and refinements in threat aircraft
bearing information.
FIG. 12 shows a processing system arrangement 120 according to the
present invention for generating bearing information from received
transponder reply signals. The four logarithmically compressed
output signals of receiver channels 81-84 (FIG. 8) are coupled to
A/D converters 85-88, respectively, for generating digitized
signals from the compressed output signals. The outputs of A/D
converters 85 and 86 are compared by comparator 89 for determining
the F/A polarity bit. Similarly, the outputs of A/D converters 87
and 88 are compared by comparator 90 for determining the R/L
polarity bit.
The outputs of A/D converters 85 and 86 are input to
.vertline.F-A.vertline. generator 91 for generating a signal
representing the magnitude of the difference between the
forward-channel and the aft-channel of the system. The outputs of
A/D converters 87 and 88 are input to .vertline.R-L.vertline.
generator 92 for generating a signal representing the magnitude of
the difference between the right-channel and the left-channel of
the system. The respective outputs of comparators 89 and 90 and of
the difference generators 91 and 92 are input to memory 93 where a
bearing look-up table is stored. The polarity bits and the
difference magnitudes applied to memory 93 cause memory 93 to
generate bearing information in a well-known manner at output
94.
Converters 85-88, comparators 89 and 90, difference generators 91
and 92, and memory 93 can be embodied as well-known, readily
available individual dedicated integrated circuits, or be a single
application specific integrated circuit (ASIC). Further, the
functions of the comparators and difference generators of the
arrangement shown in FIG. 12 can be embodied by a microprocessor
having sufficient speed for performing all of the necessary
functions and calculations for generating the input signals for
memory 93.
In FIG. 12, A/D converters 85-88 are shown digitizing the video
signals output from the respective receivers 81-84. When processing
system 120 is configured as shown in FIG. 12, A/D converters 85-88
convert the analog signals output from receivers 81-84 to digital
signal as soon as possible in the signal path so that analog-type
circuit design considerations, such as DC-offset and gain errors,
are minimized.
Processing system 120 can also be configured with comparators 89
and 90 and difference generators 91 and 92 connected directly to
the video outputs of receivers 81-84 with the outputs of the
difference generators being converted to a digital signal by A/D
converters. That is, comparator 89 and difference generator 91 can
both be connected directly to the video outputs of receivers 81 and
82, and comparator 90 and difference generator 92 can both be
connected directly to the video outputs of receivers 83 and 84. In
this configuration, difference generators 91 and 92 would be
required to generate difference signals by operating on the analog
output signals output from receivers 81-84. A separate A/D
converter would be connected to the respective outputs of the
difference generators for producing digital signals. A processing
system 120 configured in this manner would be similar to system 120
shown in FIG. 12, except A/D converters 85-88 would be omitted and
separate A/D converters would be connected between difference
generator 91 and memory 93 and difference generator 92 and memory
93.
As will be apparent to those skilled in the art, the signal
processing described herein, comparing, differencing, the table
look up etc. can be implemented by either discrete circuitry or a
software driven processor. If the latter alternate is selected then
the antenna signal processor may be the same processor which
records reply signals and processes those signals as described in
the patents cited above. On the other hand there may be reasons for
using an antenna signal processor which is separate from the reply
signal processor described in the cited patents.
The antenna array of the present invention (FIG. 6) has identical
transmitting and receiving field patterns (antenna reciprocity
theorem). Consequently, directional transmissions are achieved by
selectively driving each of the four output ports of the quadrature
hybrid circuits (two output ports 27 and 28 per each hybrid circuit
20), while terminating the other three ports that are isolated from
the driven port in the characteristic impedance of the system.
Typically, PIN diode switches are used for selecting the active
(driven) port when transmitting. A duplexer can be used in a
well-known manner for transmitting and receiving signals using a
common antenna array.
FIG. 13 shows a transmitter system 130 which can be coupled in a
well-known manner through switches SW to the output ports 27 and 28
of the quadrature hybrid circuits 20 of the present invention.
Switches SW are preferably PIN diode switches. Transmitter system
130 generates a conventional transponder interrogation signal which
is coupled to the monopole antenna elements array of the present
invention for directionally transmitting the transponder
interrogation signal. To transmit a directional signal, the
amplitudes of the signals applied to the output ports 27 and 28 of
the quadrature hybrid circuits need to be sequentially selected by
switches SW in a well-known manner. The actual respective signal
magnitudes for transmitting in a particular direction will be
proportionally the same as the magnitudes of a signal received from
the same direction.
By using the antenna array of the present invention for
transmitting, threat aircraft transponders can be directionally
interrogated without dependence on ground radar interrogations. By
actively interrogating threat aircraft transponders, distances to
threats can be measured accurately by measuring the time of arrival
of a transponder reply signal after an interrogation signal is
transmitted.
While the present invention has been described in connection with
the illustrated embodiments, it will be appreciated and understood
that modifications may be made without departing from the true
spirit and scope of the invention.
* * * * *